Since the discovery of X-rays, permanent records of X-ray patterns have been made using a photographic emulsion. X-ray intensifying screens are generally used in conjunction with photographic film and serve to enhance the image formed on the film. Phosphors which are the active components of such screens convert X-radiation into photons which are more readily captured by a photographic emulsion to provide a permanent record. The phosphors should be good absorbers of X-rays of the energy used, (typically 30 to 100 KeV); they should emit light strongly in the region of the spectrum to which the film is sensitive and they should provide sharp, undistorted film images. Ideally, this light emission is all fluorescent or "prompt" emission with little or no delayed emission (sometimes called "afterglow", "phosphorescence" or "lag").
Another method of recording X-ray images is disclosed in U.S. Pat. No. 3,859,527. A temporary storage medium, for example a photostimulable phosphor panel is exposed to incident X-rays and, as a result, has temporarily stored therein an image in the form of trapped electrons and holes, the image being representative of the incident pattern. At some interval after exposure, a beam of infrared or visible light scans the panel to stimulate the release of the stored energy as light so that the emitted light can be detected and, for example, converted to an electrical signal which can then be processed to produce a visible image. For this purpose, the phosphor should store as much of the incident energy as possible until stimulated by the scanning beam. Ideally there would be no prompt emission or lag, and all emission would occur upon subsequent stimulation.
Europium-doped barium fluorohalides, BaFCl:Eu and BaFBr:Eu, have been used in conventional commercial X-ray intensifying screens. Because of its greater density and higher speed, BaFBr:Eu would be preferred for use as an intensifying screen phosphor over BaFCl:Eu, but historically BaFBr:Eu suffered from excessive "lag" relative to BaFCl:Eu. Areas of active research to improve BaFBr:Eu for use in X-ray intensifying screens have been the development of processes for preparing the phosphors or the identification of additives for the phosphors that improve the properties required to use in intensifying screens, that is the increase in prompt emission and the reduction of "afterglow" or "lag".
BaFBr:Eu is also the preferred phosphor for use in photostimulable storage panels (U.S. Nos. 4,239,968, 4,400,619, 4,535,237, vide infra). Research has been directed towards increasing the amount of incident energy stored and released upon subsequent photostimulation with the ultimate goal of minimizing the amount of radiation to which the subject is exposed.
U.S. No. 4,239,968 discloses a method for recording a radiation image using a photostimulable phosphor of the type disclosed in U.S. No. 4,336,154, infra, the phosphor being at least one selected from alkaline earth metal fluorohalides (Ba.sub.1-x M .sub.x.sup.II)FX:yA wherein M.sup.II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd, X is at least one of the halogens Cl, Br, and I, A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, x and y are numbers satisfying the conditions 0.ltoreq.y.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2 and the wavelength of the stimulation light is not less than 500 nm nor more than 1100 nm. No preparative method for the phosphor is disclosed.
U.S. No. 4,336,154 discloses a photostimulable boron-containing phosphor which provides "heretofore unattainable high luminance". The phosphor is of the formula (Ba.sub.1-x M.sub.x.sup.II)F.sub.2. aBaX.sub.2 :yEu,zB, wherein M.sup.II and X are the same as in the aforesaid U.S. No. 4,239,968 except that M.sup.II can also be Be, and a, x, y and z satisfy the respective conditions 0.5.ltoreq.a.ltoreq.1.25, 0.ltoreq.x.ltoreq.1, 10.sup.-6 .ltoreq.y.ltoreq.2.times.10.sup.-1. Luminances exemplified are up to 1.85 times those of boron-free phosphors at stimulating light wavelengths of 450 to 800 nm. It also is disclosed that when an ammonium halide (NH.sub.4 X) is used as one of the raw materials for the phosphor, there may be times when the halogen (X) is present in excess of the stoichiometric amount in the mixture of raw materials. This excess halogen (X) is expelled from the reaction system in the form of NH.sub.4 X during firing. The phosphors are prepared by heating a mixture of the raw materials (e.g., BaF.sub.2, M.sup.II F.sub.2, BaX.sub.2 or NH.sub.4 X, a europium compound and a boron compound) at 600.degree. to 1000.degree. C. for 1 to 6 hours. The heating may be carried out in air, but it is desirable to carry it out in a neutral atmosphere such as argon or nitrogen, or in a weakly reducing atmosphere such as one containing a small amount of hydrogen gas.
European Patent Application No. 0 083 085 discloses the preparation of a photostimulable phosphor represented by the formula BaFX.sub.x NaX':aEu.sup.2+ wherein X and X' each designate at least one of Cl, Br, and I, x is a number satisfying 0&lt;x.ltoreq.10, and "a" is a number satisfying 0&lt;a.ltoreq.0.2. The first of two firing steps is carried out in a weakly reducing atmosphere, such as nitrogen gas containing a small amount of hydrogen gas and the second of two firings is carried out in a neutral atmosphere such as nitrogen or argon gas.
U.S. No. 4,608,190 discloses phosphors that corresponding to the formula Ba.sub.1-(a+b+c) K.sub.(a+c).sup.Eu.sub.b F.sub.1-a X.sub.1-c f.sub.(a+c) where X=at least one of Cl and Br, f=anion deficiencies, a=0 to about 0.05, c=0 to about 0.05, a+c=0.0005 to about 0.05, and b=about 0.001 to about 0.02. The anion deficiencies are generated by heating a europium-doped barium fluorohalide, which is prepared by any commonly used method for utility in a conventional X-ray intensifying screen, in a hydrogen-containing inert gas atmosphere to remove some of the fluorine, chlorine and/or bromine.
U.S. No. 4,100,101 discloses phosphors that correspond to the formula Ba.sub.1-x Sr.sub.x FH:yEu wherein x is from about 0 to about 0.5, y is from about 0.001 to about 0.100 and H is a halogen selected from chlorine and bromine wherein the Eu is a mixture of Eu.sup.+2 and Eu.sup.+3, the Eu.sup.+3 being present in at least an effective and detectable amount. Phosphors of this invention are prepared in a non-reducing atmosphere. Cited utility of the phosphors is in X-ray intensifying screens. There is no disclosure nor suggestion that the phosphors disclosed in this patent contain oxygen. There is no disclosure nor suggestion that the phosphors disclosed in this patent contain anion deficiencies. There is no disclosure nor suggestion that the process disclosed in this patent can be used to prepare oxygen-containing phosphors or anion vacancy-containing phosphors. There is no disclosure nor suggestion that the phosphors disclosed in this patent would possess utility in photostimulable image storage panels.
A.L.N. Stevels et al., Philips Research Reports, 30, 277 (1975) in a discussion of the preparation of BaFCi:Eu and BaFBr:Eu phosphors for use in X-ray intensifying screens state that "even small traces of oxygen in the starting materials or in the gas flow during heating were fatal for both the light output and the afterglow of the sample." No indication of any beneficial effect of oxygen in the gas flow was disclosed nor was any suggested.
In U.S. Pat. No. 4,515,706, there are disclosed rare earth oxyfluoride barium fluoride halide phosphors of the formula aREOF.(1-a)Ba.sub.(1-x) M.sub.x FX:Eu.sub.y, where RE is one element selected from the group consisting of Lu and Y, M is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd, X is at least one element selected from the group consisting of Cl, Br, and I, a is greater than zero but less than or equal to 0.6, x is in the range from 0 to 0.5, inclusive, y is in the range from 10.sup.-6 to 2.times.10.sup.-1, inclusive, and a is an amount such that the phosphor exhibits a higher luminous intensity than said phosphor without REOF when stimulated by light having a wavelength of 550 nm after being exposed to X-ray radiation. It is stated that "calcination may be carried out in air, but preferably the atmosphere is a reducing atmosphere such as N.sub.2 (sic) or a mixture of N.sub.2 and H.sub.2." There is no disclosure nor suggestion that curopium-doped barium fluorobromide phosphors might benefit from being fired in air.
All of the above references that refer to europium-doped barium fluorobromide phosphors invariably teach the use of hydrogen or hydrogen-containing atmospheres during firing to achieve reasonable photostimulability. It is therefore surprising and unexpected that use of an atmosphere comprising 1-2% oxygen in nitrogen during firing would yield a useful, photostimulable phosphor with superior performance characteristics. In this phosphor composition, it is believed that the oxygen provides a site for hole trapping and the vacancy will trap the electrons from electron hole pair generation upon X-ray stimulation.